The crosslinking (curing) of polymers is an important commercial activity, useful, for example, in elastomers, in coatings, and in thermoset materials such as are used for electronics. Controlling when and under what conditions crosslinking takes place is usually critical since once a polymer is crosslinked it is usually not “workable”, that is it may not be reshaped. In some applications, such as coatings and electronic applications it may be desirable or even mandatory that no lower molecular weight compounds be volatilized during or after the crosslinking of the polymers, so as not to contaminate sensitive equipment such as electronics, and/or to pollute the environment, as in the case of coatings.
Numerous ways have been found to avoid the production of volatile compounds during curing. For example the reaction of epoxy groups with other groups such as hydroxyl groups may accomplish this result, but it is sometimes difficult to control after the ingredients are mixed and/or higher temperatures are required. To avoid these types of problems, especially in coatings which often must be cured under conditions close to ambient conditions and which often must be stable for long periods before curing, other solutions have been found, such as the use of spiroorthoesters, see for instance World Patent Application 9731073. However new and/or improved methods of crosslinking polymers are always of interest.
For coatings, basecoat-clearcoat systems have found wide acceptance in the past decade as automotive finishes. Continuing effort has been directed to such coating systems to improve the overall appearance, the clarity of the topcoat, and the resistance to deterioration. Further effort has been directed to the development of coating compositions having low volatile organic content (VOC). A continuing need exists for coating formulations which provide outstanding performance characteristics after application, and particularly mar-resistance and resistance to environmental etching. Heretofore, mar-resistant coatings were attained by softening the coating, which depreciates other performance characteristics. The instant invention helps overcome this problem.
In repairing damage, such as dents to auto bodies, the original coating in and around the damaged area is typically sanded or ground out by mechanical means. Some times the original coating is stripped off from a portion or off the entire auto body to expose the bare metal underneath. After repairing the damage, the repaired surface is coated, preferably with low VOC coating compositions, typically in portable or permanent low cost painting enclosures vented to atmosphere to remove the organic solvents from the freshly applied paint coatings in environmentally safe manner. Typically, the drying and curing of the freshly applied paint takes place within these enclosures. Furthermore, the foregoing drying and curing steps take place within the enclosure to also prevent the wet paint from collecting dirt in the air or other contaminants.
As these paint enclosures take up significant floor space of typical small auto body paint repair shops, these shops prefer to dry and cure these paints as fast as possible. More expensive enclosures are frequently provided with heat sources, such as conventional heat lamps located inside the enclosure to cure the freshly applied paint at accelerated rates. Therefore, to provide more cost effective utilization of shop floor space and to minimize fire hazards resulting from wet coatings from solvent based coating compositions, there exists a continuing need for low VOC fast curing coating formulations which cure under ambient conditions while still providing outstanding performance characteristics, particularly overcoming mar problems and resistance to environmental etching.
Spiroorthocarbonates have been used in a variety of crosslinkable polymeric systems, usually helping to crosslink the system by cationic polymerization of the spiroorthocarbonate groups and sometimes (co)polymerizing other groups such as epoxides, see for instance World Patent Application 0040280, U.S. Pat. No. 5,808,108, and C. Pan, et al., Polym. Int., Vol. 49, p. 74–80 (2000). None of these references describe the crosslinking of spiroorthocarbonate containing compositions via hydrolysis of the spiroorthocarbonate groups.